Even the experts cannot give a reliable answer as to which plant diseases remain active after composting and which do not, because the behavior of the various pathogens in compost has hardly been scientifically studied so far. The central question is: Which fungal pathogens form such stable permanent spores that they are still contagious even after several years, and what is allowed on the compost?
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Disease transmission through compost
The so-called soil-borne harmful fungi are particularly resistant. These include, for example, the pathogens of cabbage hernia and various wilt fungi such as Fusarium, Verticillium and Sclerotinia. The fungi live in the soil and form permanent spores that are highly resistant to drought, heat and decomposition processes. Plants with diseased discoloration, rotting, or growths at the base of the stem should generally not be composted: Pathogens that have survived the rotting process are distributed in the garden with the compost and may infect new plants again directly through the root.
Leaf fungi die in the compost
In contrast, plant parts infected with leaf fungi such as rust, powdery mildew or scab are relatively harmless. They can almost always be composted without hesitation because, with a few exceptions (for example powdery mildew), they do not form stable permanent spores. Moreover, many pathogens can only survive on living plant tissue. Because the light spores usually spread with the wind, it is almost impossible to prevent a new infection anyway – even if you meticulously sweep up all the leaves in your own garden and dispose of them with the household waste.
Viral diseases such as the widespread mosaic virus in cucumbers are also not a problem, because hardly any virus is robust enough to survive in the compost. Things are somewhat different with bacterial infections such as fire blight. The infected branches of pears or quinces should never be put into the compost, as they are highly contagious.
Heat death in the compost
When garden waste is composted professionally, so-called hot rotting occurs after only a few days, during which temperatures above 70 degrees can be reached. Under such conditions, most pests and weed seeds are killed. To ensure that the temperature rises accordingly, the compost must contain a lot of nitrogen-rich material (for example, lawn clippings or horse manure) and at the same time be well aerated. Before spreading the finished compost, remove the outer layer and put it back on again. It does not heat up as much during the rotting process and therefore may still contain active pathogens.
By the way, scientists have found that the high temperature is not the only reason for the natural disinfection of the waste. During decomposition, some bacteria and radiation fungi produce antibiotic-active substances that kill pests.
Pests in compost
You should also not completely disregard pests: Horse chestnut leaves infested with leaf miners, for example, do not belong on the compost. The pests fall to the ground with the leaves and leave their tunnels after a few days to overwinter in the soil. Therefore, it is best to sweep up the autumn leaves of horse chestnuts daily and dispose of them in the organic waste garbage can.
An overview of pests and diseases
In summary, plants and plant parts that are affected by leaf diseases or pests may be composted, with a few exceptions. Plants with pathogens that survive in the soil should not be put in the compost.
Unproblematic in the compost are …
- late blight
- Pear lattice rust
- Powdery mildew
- Pointed drought
- Rust diseases
- Apple and pear scab
- Leaf spot diseases
- curl disease
- almost all animal pests
Problematic are …
- Cabbage hernia
- Root gall blight
- Fusarium wilt
- Sclerotinia
- Carrot, cabbage and onion flies
- leaf miner moths and flies
- Verticillum wilt
